Conventionally, semiconductor devices having desired electric circuitry or desired electric elements have been formed by forming a multi-layer film structure on a semiconductor substrate and then performing etching or the like.
FIG. 5 is a sectional view of a conventional semiconductor laser device which is an example of a semiconductor device. This semiconductor laser device has the ridge 12′ in a stripe shape, and light and electric current are confined by this ridge 12′. As a result, this semiconductor laser device exhibits good lasing ability with a simple structure, and is used in various fields such as optical communication, optical recording and measurements, for light-emitting devices and optical fiber amplifier excitation apparatus.
This semiconductor laser device has the lower clad layer 3, active layer 4, upper clad layer 5, contact layer 9 and insulating film 6 sequentially formed on the upper face of semiconductor substrate 2. A part of the upper clad layer 5, contact layer 9 and the insulating film 6 together form ridge 12′. Moreover, the negative electrode 1 is formed on the lower face of the semiconductor substrate 2, and the positive electrode 17 is formed on the upper face of the semiconductor laser device.
This conventional semiconductor laser device is manufactured with a method that is shown in FIGS. 6A to 6C and FIGS. 7A to 7C. As shown in FIG. 6A, the lower clad layer 3, active layer 4, upper clad layer 5 and the contact layer 9 are sequentially formed on one surface of the semiconductor substrate 2. Then, the resist 10 is applied to form a film on the contact layer 9.
Thereafter, as shown in FIG. 6B, the contact layer 9 and the upper clad layer 5 are etched by to thereby form the ridge 12′ having the same width as that of the resist 10. Subsequently, as shown in FIG. 6C, the resist 10 is removed, and the insulating film 6 is formed on the exposed surfaces of the upper clad layer 5 and the contact layer 9. Thus, the insulating film 6 completely covers the ridge 12′.
Subsequently, as shown in FIG. 7A, the resist 11 is applied on the insulating film 6. This resist 11 is applied in such a manner that it covers the ridge 12′. Although not shown in the drawing, another resist is formed above the resist 11 thereby making the surface above the ridge 12′ and its periphery flat. Thereafter, as shown in FIG. 7B, photolithography and oxygen plasma ashing processing are performed thereto, to remove the resist 11 on the upper face of the ridge 12′ and the peripheral portion thereof, up to the height of the insulating film 6 of the ridge 12′, to thereby expose the insulating film 6 on the upper face of the ridge 12′.
Subsequently, as shown in FIG. 7C, the insulating film 6 on the upper face of the ridge 12′ is removed by plasma etching processing, to thereby expose the contact layer 9. Then, the resist 11 is completely removed, and the positive electrode 17 (see FIG. 5) is deposited on the entire upper face including the side of the ridge 12′. Next, the thickness of the substrate is reduced by milling and polishing the lower face of the substrate. Finally, the negative electrode 1 (see FIG. 5) is deposited on the lower face of the semiconductor substrate 2. Thus, the conventional ridge-type semiconductor laser device is obtained.
The positive electrode 17 is typically formed using several photolithographic steps, a dry deposition step, and a plating step. Specifically, a first photolithographic step is used to form a lift-off mask which does not cover the area where electrode 17 is to be formed. This step typically includes the step of using an ionic developer solution (e.g., alkaline solutions such NaOH, KOH, and tetra-methyl-ammonium hydroxide, TMAH) to develop a pattern in the lift-off mask. Next, a dry deposition process (e.g., sputtering) is used to deposit a thin metallic adhesion layer, followed by a thin metallic barrier layer which resists the diffusion of gold into the adhesion layer. The portions of the metal layer that are formed on the lift-off mask, which are not wanted in the finished device, are removed by a second photo-lithographic step which removes the underlying lift-off mask by exposing it to an organic solvent (e.g., washing it in the solvent).
Next, a thin gold seed layer for a subsequent electroplating step is usually deposited over the surface of the wafer. However, this step can be omitted if gold can be directly electroplated onto the barrier layer. A third photo-lithographic step is then used to form a patterned plating mask over the surface of the substrate such that the top of electrode 17 is exposed. This photo-lithographic step includes the use of an ionic developer solution to develop a pattern in the plating mask layer. Next, a thick layer of gold is typically plated over the top of electrode 17 using an ionic plating solution and the plating mask. Then the plating mask is removed by washing in an organic solvent (a fourth photo-lithographic step). If desired, a brief exposure to a gold etchant can be used to remove the seed layer which was previously covered by the plating mask.
Similar steps are used to form the negative electrode 1, including exposure to one or more ionic developer solutions and one or more organic solvents. The milling process used to thin the substrate also uses organic solvents, and may sometimes use ionic developer solutions.
In the conventional semiconductor laser device, however, when the positive electrode 17 and the negative electrode 1 are formed using the photolithography and plating steps described above, there often occurs an erosion of the contact layer 9. When the contact layer 9 is eroded, the current channel of the obtained semiconductor laser device becomes narrow and thereby the electric resistance increases. As a result, there is a drawback that the optical output of the semiconductor laser device decreases. If the erosion is severe, most of the contact layer 9 and the upper clad layer 5 are affected, thereby the optical output further decreases.
As part of making their invention, the inventors have found that the plasma etching process shown in FIG. 7C causes the top edges of insulating film 6 to have inclined surfaces 23 instead of flat surfaces, as shown in FIG. 8, and as shown in an enlarged view in FIG. 11. The inclined surfaces 23 create small gaps 24 between insulating film 6 and contact layer 9 of the ridge, which create discontinuities in the surface over which electrode layer 17 is deposited. With experiments on several samples conducted as part of making their invention, the inventors, the inventors have further found that an erosion 22 is observed near the upper portion of the contact layer 9 near these small gaps 24. They have also found that a crack 21 extending all the way through the electrode layer 17 is generally observed near the erosion 22 at the boundary between the positive electrode 17 and the insulating film 6. This is shown at the left side of the device in FIG. 8. We have shown on the right side of the device of FIG. 8 the case where the crack does not extend all the way through the electrode layer. Although FIG. 8 shows the erosion 22 only on the left side of the semiconductor laser device, similar erosion is observed on the right side as well.
Moreover, as part of making their invention, the inventors have found that the ionic solutions used during the photolithography and plating steps infiltrates to the contact layer 9 via the crack 21 and erodes the contact layer 9 due to electrochemical reaction.
The present invention is focused on reducing, and preferably eliminating, this undesirable erosion.